Connectorized nano-engineered optical fibers and methods of forming same

ABSTRACT

Connectorized nano-engineered optical fibers and method for forming them are disclosed. The methods include heating a mid-span bare fiber portion of the nano-engineered fiber to collapse the airlines therein so as to form an airline-free portion. The fiber is then inserted into a ferrule channel so that the fiber end protrudes beyond the ferrule end face, but with the airline-free portion positioned at the ferrule end face. The fiber is then cleaved at or near the ferrule end face in the airline-free portion, and the new fiber end face polished to create a solid fiber end face that coincides with the ferrule end face. The methods result in at most only minimal changes to the mode field diameter (MFD) and/or to the outer cladding diameter, which is essential in forming a connectorized nano-engineered fiber that can connect to like-size nano-engineered or non-nano-engineered fibers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119(e)of U.S. Provisional Application Ser. No. U.S. Parent ProvisionalApplication Ser. No. 60/927,430 filed on U.S. Parent ProvisionalApplication Filing Date of May 3, 2007, which application isincorporated by reference herein.

This application is also related to U.S. patent application Ser. No.11/595,365, entitled “Method of splicing an optical fiber with holes inthe cladding,” filed in the United States on Nov. 9, 2007, whichapplication is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to connectorized optical fibers,and more specifically, to methods for collapsing airlines in thecladding of nano-engineered optical fibers that seeks results in aconnectorized fiber with at most only minimal changes to the mode-fielddiameter and/or the outer cladding diameter of the fiber prior toconnectorization.

2. Technical Background of the Invention

Optical fiber connectors are used to terminate the ends of opticalfibers. Optical fiber connectors enable rapid connection anddisconnection of optical fibers as compared to fusion splicing.Connectors serve to align the cores of mating optical fibers so thatlight can pass between them with minimal loss (attenuation), and providea mechanical coupling to hold the mating fibers together. In the earlydays of fiber optic systems, the use of connectors was problematicbecause poor connections introduced attenuation, and theconnectorization process was time-consuming and required highly trainedtechnicians. However, manufacturers have since standardized andsimplified optical fiber connectors, thereby contributing to theirincreased use in fiber optic systems. The increased use of connectorshas greatly contributed to new uses and applications for fiber opticsystems, including new and creative deployments in buildinginfrastructures.

Attendant with the increased use of fiber optic systems are issuesrelating to deploying optical fiber cables wherein the cables need to bebent to accommodate the geometry of a pre-existing structure orinfrastructure. Improper handling and deployment of a fiber optic cablecan result in macrobending losses, also known as “extrinsic losses.” Inray-optics terms, severe bending of an optical fiber can cause theangles at which the light rays reflect within the fiber to exceed thecritical angle of reflection. Stated in electromagnetic-wave terms, thebending causes one or more of the guided modes of the optical fiber tobecome leaky modes wherein light escapes or “leaks” from guiding regionof the fiber. Such bending losses can be prevented by observing theminimum bend radius of the particular optical fibers and optical fibercables that carry the optical fibers.

Because deploying fiber optic cables typically involves bending one ormore of the cables at some location, advanced optical fibers have beendeveloped that have improved bend performance properties. Enhanced bendperformance allows for fiber optic cables to be deployed in a greaternumber of locations that might not otherwise be accessible due to thebending limits of a conventional fiber optic cable.

One type of bend-performance optical fiber is a “nano-engineered” fiberthat utilizes small holes or voids (“airlines”) formed in the opticalfiber. Nano-engineered fibers operate using basically the samewave-guiding principles as ordinary optical fibers wherein the light isguided in the core by the index difference between the core andcladding, with the exception that the nano-engineered region enhancesthe fibers light-carrying ability even when severely bent. However,while nano-engineered bend-performance fibers offer a significantincrease in the minimum bend radius, there are some shortcomings when itcomes to connectorizing such fibers because of the voids present at theend of a cleaved fiber. For example, contaminants can fill the fibervoids at the fiber end face and ingress into the fiber, thereby reducingthe efficiency of the connection. One such contaminant is moisture.Other contaminants include micro-debris generated at the connector endface during the connector polishing processes, such as mixtures ofzirconium ferrule material and silica glass removed during polishing,abrasives from polishing films, and deionized water. These contaminantsmay become trapped or embedded in the airlines at the connector endface. Due to the various forces and attendant heat the connector endexperiences during the polishing process, it is extremely difficult toremove the contaminants once they are in place. In addition,contamination in the fiber that is freed during operation and/orhandling of the fiber and that moves across the connector end face intothe fiber core region may also increase signal attenuation.

While cleaning the fibers after the connector polishing step may bepossible using methods such as ultrasonic cleaning, this is most oftenonly a temporary fix because the fiber remains at risk of futurecontamination because the fiber end face still has open voids. While thefiber end face may be treated using UV or heat cured materials such asadhesives or epoxies to fill the fiber voids, the material used to sealthe fiber may polish at a different rate than the optical fiber, causingindentations or protrusions on the connector end face. These featuresmay potentially interfere with the physical contact of the connector endfaces during mating or, in the case of indentations, may serve as areasfor debris or other contaminants to collect and adversely impactconnector performance.

One approach for reducing or eliminating the risk of contamination of anano-engineered fiber in the connectorization process is to seal the endof the fiber with a sealant material. However, this will cause a changein the mode-field diameter of the fiber if there is an index mismatchbetween the fiber and the sealant material. Since the most efficientoptical coupling is associated with matching the mode-field diameter ofthe fibers being coupled, a change in the mode-field diameter of onefiber relative to another can adversely affect the splicing/couplingefficiency.

Another approach for reducing or eliminating the risk of contaminationof nano-engineered fibers in the connectorization process is to use heatto collapse the airlines at the end of the fiber. However, thispotentially can lead to several problems. The first is that fusing theend of the fiber tends to change the shape of the fiber and may bedifficult to control in a manufacturing environment. Generally, thefused end tends to become bulbous and often will not fit into aconnector ferrule. The second is that fusing the end and thenconnectorizing the end can lead to damaging the end as the fiber end isinserted into the ferrule during the connectorization process.

SUMMARY OF THE INVENTION

In various embodiments, the present invention provides for theelimination of or the prevention of trapped contamination in airlinesaround the fiber core of a nano-engineered fiber by collapsing theairlines prior to connectorization in a manner that results in at mostonly minimal changes to the mode-field diameter (MFD) for single-modefibers, or the core diameter for multi-mode fibers, and/or to the outercladding diameter of these types of fibers.

In one embodiment, the present invention includes a method of forming aconnectorized nano-engineered optical fiber. The method includesproviding a nano-engineered fiber and preparing the fiber at a mid-spanportion by stripping the buffer or coating and optionally cleaning theresulting bare fiber to remove residue coating or buffer. The mid-spanportion length should be sufficient for installation into an opticalconnector while allowing the bare fiber to extend completely through thelength of the connector ferrule. With the fiber coating and/or bufferremoved, in one embodiment the fiber is positioned in a fusion splicersuch that an arc is applied to the portion of the mid-span region of thefiber that will eventually be positioned at the end of the connectorferrule after the installation and polishing processes are completed.

The preferred region of the fiber to be processed is a mid-span portion,which includes a portion of the fiber near the end but at least tenfiber diameters away from the end so that the airlines are collapsedsome distance away from the end of the fiber to avoid theabove-mentioned deleterious end-effects. The electric arc is capable ofcollapsing the airlines in the fiber to form an airline-free portion.The current used to form the electric arc is preferably in the rangefrom about 12 mA to about 16 mA for a single fiber. While the currentneeds to be sufficiently great to collapse the airlines, it must also besufficiently low to avoid damaging effects on the fiber, such as meltingand deformation. In particular, the method of the present invention iscarried out in a manner that results in at most only minor changes tothe MFD and/or to the outer cladding diameter in the airline-freeportion as compared to the unprocessed portions of the fiber.

Once the airlines have been collapsed, the optical fiber is installedinto an optical connector with some length extending beyond the end faceof the connector ferrule but with the airline-free region positionedcentrally at the ferrule end face. As part of the connectorizationprocess, the optical fiber is then precision cleaved at or near theferrule end face so that the new fiber end is within the airline-freeregion. Following the precision cleave, the optical connector is buffedand polished using standard polishing techniques known in the art sothat the now-solid fiber end face coincides with the ferrule end face.

Localized heating may be generated in various ways including, but notlimited to, an electric arc generated between two electrodes (similar tothat used in most fusion splicers), a heated filament, a flame or alaser, among others. Connector types include, but are not limited to,so-called SC/APC, SC/UPC, FC/APC, FC/UPC, LC/APC, LC/UPC and MT basedconnectors. The nano-engineered fibers for which the methods of thepresent invention are suitable include, but are not limited to,individual nano-engineered fibers and multi-fiber ribbons for use ineither single fiber or multiple fiber connectors. As discussed below,the methods of the present invention do not apply to photonic crystalfibers because the method causes significant changes to the MFD and/orthe outer cladding diameter of such fibers, which prevent properconnectorization with other like fibers, including standard opticalfibers such as Corning SMF28e™ optical fiber.

Additional features and advantages of the invention are set out in thedetailed description that follows, and in part will be readily apparentto those skilled in the art from that description or recognized bypracticing the invention as described herein, including the detaileddescription that follows, the claims, as well as the appended drawings.It is to be understood that both the foregoing general description andthe following detailed description present exemplary embodiments of theinvention, and are intended to provide an overview or framework forunderstanding the nature and character of the invention as it isclaimed. The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated into and constitutea part of this specification. The drawings illustrate variousembodiments of the invention, and together with the detaileddescription, serve to explain the principles and operations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention are better understood when the following detailed descriptionof the invention is read with reference to the accompanying drawings, inwhich:

FIG. 1 is a side view of a section of nano-engineered fiber;

FIG. 2 is a cross-sectional view of the nano-engineered fiber of FIG. 1as viewed along 2-2 therein, along with an example effective refractiveindex profile for the various fiber regions;

FIG. 3 is a plot of an example effective refractive index profile for anexample nano-engineered fiber that has a varying-index core;

FIG. 4 is a close-up view of an end of a nanostructure fiber coupled toa light source, with the numerical aperture (NA) of the optical fiberbeing greater than that of the light source;

FIG. 5A is a side view of a nano-engineered fiber with a bare sectionformed at a mid-span location;

FIG. 5B is similar to FIG. 5A, and shows the nano-engineered fiber cutat one end of the mid-span location to form a fiber end face;

FIG. 5C is a similar to FIG. 5B, and shows the nano-engineered fiberbeing subject to an electric arc that collapses the airlines to form anairline-free region within the fiber but at a distance d from the fiberend face;

FIG. 5D is similar to FIG. 5C, and shows the step, after theairline-free region is formed, where the fiber end section is insertedinto a connector ferrule with a portion of the fiber end sectionprotruding beyond the ferrule end face;

FIG. 5E is a close-up view of the ferrule end showing the ferrule endface and the fiber end section protruding from the ferrule channelbeyond the ferrule end face wherein the airline-free region ispositioned at the fiber end face plane;

FIG. 5F is similar to FIG. 5D and shows the fiber after it is cleavednear the ferrule end face and thus at the airline-free region, and ispolished so that the polished fiber end coincides with the ferrule endface and is airline free (i.e., solid);

FIG. 5G and FIG. 5H are similar to FIG. 5B and FIG. 5C and illustrate anexample embodiment of the invention as applied to a fiber ribbon;

FIG. 6 is a close-up cross-sectional view similar to FIG. 5E,illustrating how the airlines terminate to form the solid fiber endface;

FIG. 7A is a side view similar to FIG. 5A illustrating an exampleembodiment of the method wherein airlines are collapsed at the mid-spanlocation via heating from an electric arc prior to cleaving bare fiber;

FIG. 7B is similar to FIG. 7A, but shows the two resulting fibersections formed by cleaving the bare fiber at the mid-span airline-freeportion formed as shown in FIG. 7A;

FIG. 7C and FIG. 7D are similar to FIG. 7A and FIG. 7B and illustrate anexample embodiment of the invention as applied to a fiber ribbon;

FIG. 8A is a plot of the relative power (dB) vs. fiber radius r (μm) fora nano-engineered fiber having a core/clad (herein referred to as “C/C”)ratio of about 0.42, wherein the plot is representative of the MFD;

FIG. 8B is same plot as FIG. 9A but for a nano-engineered fiber having aC/C ratio of about 0.33 and a smaller change in the MFD (i.e., a smallerΔMFD) as compared to the plot of FIG. 9A; and

FIG. 9 plots simulated data that represents the theoretical ΔMFD (in μm)between the processed and unprocessed regions of the nano-engineeredfiber as a function of the C/C ratio.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is now made to preferred embodiments of the invention,examples of which are illustrated in the accompanying drawings. Wheneverpossible, the same reference numbers and symbols are used throughout thedrawings to refer to the same or like parts.

Definitions and Terminology

In the description below, the “refractive index profile” is therelationship between refractive index or relative refractive index andwaveguide fiber radius. The “relative refractive index percent” isdefined as Δ_(i)(%)=[(n_(i) ²−n_(c) ²)/2n_(i) ²]×100, where n_(i) is themaximum refractive index in region i, unless otherwise specified, andn_(c) is the average refractive index of the cladding region, asdiscussed below. In an example embodiment, n_(c) is taken as therefractive index of an inner annular cladding region 32, as discussedbelow.

As used herein, the relative refractive index percent is represented byΔ(%) or just “Δ” for short, and its values are given in units of “%”,unless otherwise specified or as is apparent by the context of thediscussion.

In cases where the refractive index of a region is less than the averagerefractive index of the cladding region, the relative refractive indexpercent is negative and is referred to as having a “depressed region” ora “depressed index,” and is calculated at the point at which therelative refractive index is most negative unless otherwise specified.In cases where the refractive index of a region is greater than theaverage refractive index of the cladding region, the relative refractiveindex percent is positive and the region can be said to be raised or tohave a positive index.

An “updopant” is herein considered to be a dopant, which has apropensity to raise the refractive index relative to pure undoped SiO₂.A “downdopant” is herein considered to be a dopant, which has apropensity to lower the refractive index relative to pure undoped SiO₂.An updopant may be present in a region of an optical fiber having anegative relative refractive index when accompanied by one or more otherdopants that are not updopants. Likewise, one or more other dopants thatare not updopants may be present in a region of an optical fiber havinga positive relative refractive index. A downdopant may be present in aregion of an optical fiber having a positive relative refractive indexwhen accompanied by one or more other dopants that are not downdopants.Likewise, one or more other dopants that are not downdopants may bepresent in a region of an optical fiber having a negative relativerefractive index.

Other techniques to form depressed index regions besides the use ofdowndopants, such as through the use of microstructures, are used inexample embodiments of the present invention and are described ingreater detail below. Microstructures include, for example, non-periodicand periodic discrete microvoids occurring along the length of the fibersuch as airlines having a diameter in fiber cross-section of greaterthan 5 nm (nanometer) and less than 1550 nm (for example airlinesgreater than 5 nm with an average diameter of approximately 250 nm).

The “core-cladding ratio” is the “C/C ratio” which is a measure of theseparation between the core region and the annular hole-containingregion of a nano-engineered fiber and is given by the ratio of the(outer) radius R₁ of the core (the core radius R₁ is defined whereΔ₁=0.04% and is spaced apart from the centerline of the fiber) to theinner radius R₂ of the annular hole-containing region (which is theouter radius of the inner annular cladding region) as depictedschematically in FIG. 2, the “C/C” ratio as used herein is the ratio of“R₁/R₂.” The ratio R₁/R₂ is evident during manufacturing of the opticalfiber, and can be determined and measured after manufacturing by, forexample, optical observation of the end face of the optical fiber. Thehigher the C/C ratio, the less cladding there is for the same amount ofcore, and the closer the annular hole-containing region ring is to thecore, and vice versa.

The terms voids, holes and airlines can be used interchangeably and meana portion of the optical fiber which contains empty space or a gas

The mode field diameter (MFD) is a measure of the spot size or beamwidth of light across the end face of an optical fiber. MFD is afunction of source wavelength and the fiber geometry, i.e., fiber coreradius and fiber refractive index profile. The vast majority of theoptical power propagating in an optical fiber travels within the fibercore, with a small amount of power propagating in the cladding as anevanescent field. Mismatches in the mode field diameter can affectsplice and connector loss. The MFD is measured using the Peterman IImethod wherein, 2 w=MFD, and w²=(2∫f² rdr/∫[df/dr]² rdr), the integrallimits being 0 to ∞. A method of experimentally measuring the MFD is thevariable aperture method in the far field (VAMFF), which is described inthe article by Parton, J. R., “Improvements in the Variable ApertureMethod for Measuring the Mode-Field Diameter of a Dispersion-ShiftedFiber,” Journal of Lightwave Technology, Vol. 7, No. 8, August 1989 (pp.1158-1161), which article is incorporated by reference herein. The MFDis measured in irradiance, which is optical power per unit area (W/cm²).

For a Gaussian power distribution in a single-mode optical fiber, theMFD is measured between points at which the electric and magnetic fieldstrengths are reduced to 1/e of their maximum values, i.e., it is thediameter at which the optical power is reduced to 1/e² of the maximumpower, wherein power is proportional to the square of the fieldstrength. As used herein, MFD assumes a given wavelength (e.g., 1550 nm)unless otherwise noted.

A related concept to MFD is the “effective area” of an optical fiber,which is defined as: A_(eff)=2π(∫f² r dr)²/(∫f⁴ r dr), where r is theradial coordinate, the integration limits are 0 to ∞, and f is thetransverse component of the electric field associated with lightpropagated in the optical fiber.

In multi-mode fibers (either the step-index or the graded-indexvariety), the core diameter is typically used to measure thedistribution of the light beam exiting the fiber. The core size ismeasured optically, and in a preferred approach is defined as a diametercorresponding to a specific threshold percentage of power in the core. Astandard core diameter measurement approach is set out by theInternational Electrotechnical Commission (IEC) Standard IEC 60793-1-2(2001). For reference, see the IEC 60793-1-2 document under “Measurementmethods and test procedures—Fibre geometry,” Section C.4.2.2 Option 2,which document section is incorporated by reference herein by way ofbackground information. The IEC standard is used herein as thedefinition of core diameter for multi-mode nano-engineered fibers 10.

Note that MFD and the core diameter are related concepts that describethe distribution of light exiting the fiber based on a threshold amountof power. In the present invention, these parameters change bysubstantially the same amounts. Accordingly, the description herein andthe claims below use the term “mode field diameter” or MFD in thegeneral sense to describe the light distribution associated with eithera single-mode or a multi-mode fiber, where the MFD for a single-modefiber is defined as above, and wherein the MFD of a multi-mode fiber isdefined as the core diameter according to the aforementioned IECstandard.

In general, the MFD increases after heating the fibers, though in someinstances the MFD could decrease. Thus, changes in the MFD (denoted“ΔMFD”) refer to the absolute value of the change.

Nano-Engineered Fibers

The present invention relates to nano-engineered optical fibers(“nano-engineered fibers”) wherein nano-engineered features in the formof relatively small aperiodically arranged airlines facilitate theguiding of optical-frequency electromagnetic waves in a glass fiber corein a manner that makes the optical fiber resistant to optical loss evenwhen the fiber is bent to a small bending radius (i.e., the fiber is“bend resistant”). The nano-engineered fibers otherwise operate in thesame manner as standard optical fibers. This is in contrast to photoniccrystal fibers, which are constituted by a periodic array of relativelylarge holes formed in a dielectric medium (or by an array of dielectrictubes), wherein the guiding of optical-frequency electromagnetic wavesis dictated by allowed and forbidden photonic energy bands defined bythe array of holes. Nano-engineered fibers have airlines limited to arelative small airline-containing region wherein the air-fill percent(the area fraction of airlines to the area of the optical fiber times100%, at a pre-selected cross-section) is less than about 1% and isusually about 0.02% to about 0.2% or about 0.3%. The termnano-engineered optical fiber (also sometimes referred to herein asmicro-structured optical fiber) refers to an optical glass fibercomprising these nanometer-size features. In contrast, the holes inphotonic crystal fibers occupy a large portion of the fiber and have anair-fill percent of 5% to 50%, i.e., at least five times greater, andmore typically about two orders of magnitude greater than thenano-engineered fibers contemplated herein.

These important physical differences between these two types of fibershave practical implications in the connectorization process. Inparticular, collapsing the holes of a photonic crystal fiber necessarilycauses a significant change in the fiber size and thus the MFD, whichadversely impacts the connectorization process and the resultingconnector. Consequently, the systems and methods described herein applyonly to nano-engineered fibers and not to photonic crystal fibers.

FIG. 1 is a side view of an example embodiment of a section ofnano-engineered fiber (“nano-engineered fiber”) 10 having opposite ends12 and 14, and a centerline 16. FIG. 2 is a cross-sectional view ofnano-engineered fiber 10 as viewed along the direction 2-2 of FIG. 1.Nano-engineered fiber 10 includes a core region (“core”) 20 made up of asingle core segment having a radius R₁ and positive maximum relativerefractive index Δ₁, a cladding region (“cladding”) 30 having an annularinner cladding region (“inner cladding”) 32 with an inner radius R₁, anouter radius R₂ an annular width W₁₂ and a relative refractive index Δ₂,an annular nano-engineered or “airline containing” region 34 having aninner radius R₂, an outer radius R₃ an annular width W₂₃ and an relativerefractive index Δ₃, and an outer annular cladding region (“outercladding”) 36 having an inner radius R₃, an outer radius R₄, an annularwidth W₃₄ and a relative refractive index Δ₄. Outer annular cladding 36represents the outermost silica-based portion of nano-engineered fiber10. The total diameter of the “bare” fiber 10 is D₁₀=2R⁴. In an exampleembodiment D₁₀=125 microns, Δ₁=approximately 0.34%, R₁=approximately 4.5microns, R₂=approximately 10.7 microns, region 34 is comprised of 100holes having a mean diameter of approximately 300 nm and a maximumdiameter of <700 nm, W₂₃=approximately 4 microns and Δ₂=Δ₄=approximately0%. In another example embodiment D₁₀=125 microns, Δ₁=approximately0.34%, R₁=approximately 4.5 microns, R₂=approximately 13.6 microns,region 34 is comprised of 200 holes having a mean diameter ofapproximately 200 nm and a maximum diameter of <700 nm,W₂₃=approximately 3 microns and Δ₂=Δ₄=approximately 0%. In yet anotherexample embodiment D₁₀=125 microns, Δ₁=approximately 0.34%,R₁=approximately 4.5 microns, R₂=approximately 13.6 microns, region 34is comprised of 400 holes having a mean diameter of approximately 150 nmand a maximum diameter of <700 nm, W₂₃=approximately 3 microns andΔ₂=Δ₄=approximately 0%. In yet another example embodiment D₁₀=125microns, Δ₁=approximately 0.34%, R₁=approximately 4.5 microns,R₂=approximately 12.2 microns, region 34 is comprised of 500 holeshaving a mean diameter of approximately 120 nm and a maximum diameter of<700 nm, W₂₃=approximately 3 microns and Δ₂=Δ₄=approximately 0%. Thesefibers when measured by cable cutoff show that they are single-modedabove 1260 nm.

A protective cover 50 is shown surrounding outer annular cladding 36. Inan example embodiment, protective cover 50 includes one or more polymeror plastic-based layers or coatings, such as a buffer coating or bufferlayer.

In an example embodiment, an annular hole-containing region 34 iscomprised of periodically or non-periodically disposed holes or“airlines” 40 that run substantially parallel to centerline 16 and thatare configured such that the optical fiber is capable of single modetransmission at one or more wavelengths in one or more operatingwavelength ranges. By “non-periodically disposed” or “non-periodicdistribution,” it will be understood to mean that when one takes across-section (such as a cross-section perpendicular to the longitudinalaxis) of the optical fiber, the non-periodically disposed airlines arerandomly or non-periodically distributed across a portion of the fiber.Similar cross sections taken at different points along the length of thefiber will reveal different cross-sectional airline patterns, i.e.,various cross-sections will have different airline patterns, wherein thedistributions of airlines and sizes of airlines do not match. That is,the airlines are non-periodic, i.e., they are not periodically disposedwithin the fiber structure. These airlines are stretched (elongated)along the length (i.e. in a direction generally parallel to thelongitudinal axis) of the optical fiber, but do not extend the entirelength of the entire fiber for typical lengths of transmission fiber.Typically the airlines extend less than 10 meters, e.g., 0.2 to 1 meteror less.

As mentioned above, the nano-engineered fibers 10 suitable for use inthe present invention preferably include an air-fill percent less thanabout 1%, more preferably less than about 0.7%, and even more preferablyless than about 0.3%, and even more preferably between about 0.02% andabout 0.2%. An optical fiber suitable for use in the present inventionfurther has an average hole size of about 0.3 microns or less, such as0.15 or 0.09 microns and greater than 0.005 microns. In contrast, holeyfiber available from NTT, Japan, has an average hole size of about 12microns and an air-fill percent of >1%, and typical photonic crystalfibers have air-fill percents >5%. Thus, as mentioned above, it is thesmall airline size of the nano-engineered fibers considered herein thatallows the fibers to retain their circularity and nominally theiroriginal size when the airlines are collapsed.

Further, because of the small size of airlines 40, fibers processedusing the air hole collapsing methods of the present invention are ITU-TG.652 compliant in that a 125 μm fiber is +/−1 μm in diameter for properconnectorization processing after subjecting the fiber to the air holecollapsing method because of the less than 1% air-fill percent. Incontrast, photonic crystal fiber, after collapsing the air holestherein, has a diameter change far greater than +/−1 μm, and thus is notITU-T G.652 compliant for connectorization. Thus, the methods of thepresent invention are able to collapse airlines 40 while retaining aboutthe same cross-sectional diameter and circularity, making the fibers andmethods advantageous for mounting within a ferrule.

For a variety of applications, it is desirable for the airlines 40 ofthe nano-engineered fibers 10 considered herein to have greater thanabout 95% of and preferably all of the airlines exhibit a mean airlinesize in the cladding for the optical fiber that is less than 1550 nm,more preferably less than 775 nm, most preferably less than 390 nm andin some embodiments less than 250 nm and greater than 5 nm . Likewise,it is preferable that the maximum diameter of the airlines in the fiberbe less than 7000 nm, more preferably less than 4000 nm, more preferablyless than 1550 nm, and most preferably less than 775 nm and in someembodiments less than 300 nm. In some embodiments, the fibers disclosedherein have greater than 50 airlines, in some embodiments also greaterthan 200 airlines, and in other embodiments the total number of airlinesis greater than 500 airlines, while still in other embodiments the totalnumber of airlines is greater than 1000 airlines in a given opticalfiber perpendicular cross-section. Of course, the most preferred fiberswill exhibit combinations of these characteristics. Thus, for example,one particularly preferred embodiment of optical fiber would exhibitgreater than about 200 airlines in the optical fiber, the airlineshaving a maximum diameter less than 1550 nm and a mean diameter lessthan 775 nm, for example, the maximum diameter is less than 775 nm andthe mean diameter of about 200 nm, although useful and bend resistantoptical fibers can be achieved using larger and greater numbers ofairlines. The hole number, mean diameter, max diameter, and total voidarea percent of airlines can all be calculated with the help of ascanning electron microscope at a magnification of about 800× and imageanalysis software, such as ImagePro, which is available from MediaCybernetics, Inc. of Silver Spring, Md., USA.

Because the nano-engineered fibers 10 considered herein rely on thecore-cladding index difference to guide light, the fiber can generallyinclude germania or fluorine to also adjust the refractive index of thecore and/or cladding of the optical fiber, but these dopants can also beavoided in the intermediate annular region. The airlines (in combinationwith any gas or gases that may be disposed within the airlines) can beused to adjust the manner in which light is guided down the core of thefiber, particularly when the fiber is bent. The hole-containing regionmay consist of undoped (pure) silica, thereby completely avoiding theuse of any dopants in the hole-containing region, to achieve a decreasedrefractive index, or the hole-containing region may comprise dopedsilica, e.g. fluorine-doped silica having a plurality of airlines.

In one set of embodiments, the core region includes doped silica toprovide a positive refractive index relative to pure silica, e.g.germania doped silica. The core region is preferably airline-free.

Such fiber can be made to exhibit a single-mode behavior with a cablecutoff of less than 1400 nm, more preferably less than 1260 nm; a 20 mmdiameter macrobend induced loss at 1550 nm of less than 1 dB/turn,preferably less than 0.5 dB/turn, even more preferably less than 0.1dB/turn, still more preferably less than 0.05 dB/turn, yet morepreferably less than 0.03 dB/turn, and even still more preferably lessthan 0.02 dB/turn; a 10 mm diameter macrobend induced loss at 1550 nm ofless than 5 dB/turn, preferably less than 1 dB/turn, more preferablyless than 0.5 dB/turn, even more preferably less than 0.2 dB/turn, stillmore preferably less than 0.01 dB/turn, still even more preferably lessthan 0.05 dB/turn.

Additional description of micro-structured fibers used in the presentinvention are disclosed in pending U.S. patent application Ser. No.11/583,098, filed Oct. 18, 2006; U.S. patent application Ser. No.12/004,174, filed Dec. 20, 2007; in pending U.S. provisional patentapplication Ser. No. 60/817,863, filed Jun. 30, 2006; in U.S.provisional patent application Ser. No. 60/817,721, filed Jun. 30, 2006;in U.S. provisional patent application Ser. No. 60/841,458, filed Aug.31, 2006; in U.S. provisional patent application Ser. No. 60/876,266,filed Dec. 21, 2006; and in U.S. provisional patent application Ser. No.60/879,164, filed Jan. 8, 2007, all of which are assigned to CorningIncorporated and each application is respectively incorporated herein byreference.

The nano-engineered fibers considered herein also include multi-modenano-engineered fibers that comprise, for example, a graded-index coreregion and a cladding region surrounding and directly adjacent to thecore region, the cladding region comprising a depressed-index annularportion comprising a depressed relative refractive index, relative toanother portion of the cladding (which preferably is silica which is notdoped with an index of refraction altering dopant such as germania orfluorine). Preferably, the refractive index profile of the core has aparabolic shape. The depressed-index annular portion may comprise glasscomprising a plurality of airlines, fluorine-doped glass, orfluorine-doped glass comprising a plurality of airlines. The depressedindex region can be adjacent to or spaced apart from the core region.

The multi-mode nano-engineered fibers considered herein also exhibitvery low bend induced attenuation, in particular very low macrobending.In some embodiments, high-bandwidth is provided by a low maximumrelative refractive index in the core, and low bend losses are alsoprovided. In some embodiments, the core radius is large (e.g. greaterthan 10 microns, for example 25 microns and 31.25 microns), the corerefractive index is approximately 2% or less (e.g. 2.0%, 1.0%, 0.90% or0.50%), and the macrobend losses are low. Preferably, the multi-modeoptical fiber disclosed herein exhibits a spectral attenuation of lessthan 3 dB/km at 850 nm.

In an example embodiment, core 20 and cladding 30 are configured toprovide improved bend resistance, and single-mode operation atwavelengths preferably greater than or equal to 1500 nm, in someembodiments also greater than about 1310 nm, in other embodiments alsogreater than 1260 nm. The optical fibers provide a MFD at a wavelengthof 1310 nm preferably greater than 8.0 microns, more preferably betweenabout 8.0 and 10.0 microns.

Example Effective Index Parameters

In one set of embodiments a single-mode fiber has the following,0.30%<Δ₁<0.40%, and 3.0 μm<R₁<5.0 μm. In some embodiments, core 20 has arefractive index profile with an alpha shape, where in some embodimentsalpha is 6 or more, while in other embodiments alpha is 8 or more. In anexample embodiment of a multi-mode fiber, has the following, 12.5μm≦R₁≦40 microns. In some embodiments, 25 μm≦R1≦32.5 μm, and in some ofthese embodiments, R₁ is greater than or equal to about 25 microns andless than or equal to about 31.25 microns. In an example embodiment,core 20 preferably has a maximum relative refractive index (sometimescalled Δ_(1MAX)) of 0.5%≦Δ₁≦2.0%. In yet another embodiment, core 20 hasa maximum relative refractive index 0.9%≦Δ₁≦1.1%. In yet anotherembodiment, core 20 has a maximum relative refractive index0.4%≦Δ₁≦0.5%. Such multi-mode fibers preferably exhibit a one-turn 10 mmdiameter mandrel attenuation increase of no more than a 1 turn 10 mmdiameter mandrel wrap attenuation increase at a wavelength of 1550 nm,in dB, (also called 1×10 mm dia. bend loss at 1550 nm) of less than orequal to the product of two times (1/Δ_(1MAX))². Thus for a multi-modefiber having a core Δ_(1MAX) of 2% the 1×10 mm dia. bend loss at 1550nm≦2(1/2)²=1 dB; for a multi-mode fiber having a core Δ_(1MAX) of 1% the1×10 mm dia. bend loss at 1550 nm≦2(1/1)²=1 dB; and for a multi-modefiber having a core Δ_(1MAX) of 0.5% the 1×10 mm dia. bend loss at 1550nm≦2(1/0.5)²=4 dB.

In an example embodiment, the hole-containing region 34 has an innerradius R₂≦20 μm. In some example embodiments, 10 μm≦R₂≦20 μm. In otherembodiments, 10 μm≦R₂≦18 μm. In other embodiments, 10 μm≦R₂≦14 μm. Insome embodiments, the inner annular cladding radial width W₁₂>1 μm. Inan example embodiment, radius R₂>5 μm, and more preferably R₂>6 μm.

Again, while not being limited to any particular width, in an exampleembodiment, the hole-containing region 34 has a radial width 0.5 μm≦W₂₃,while in other example embodiments 0.5 μm≦W₂₃≦20 μm. In otherembodiments, 2 μm≦W₂₃≦12 μm. In other embodiments, 2 μm≦W₂₃≦10 μm. In anexample embodiment, the annular hole-containing region 34 has a regionalvoid area percent of less than about 30 percent and greater than 0.5percent, and the non-periodically disposed airlines have a mean diameterof less than 1550 nm. In some embodiments region 34 has a regional voidarea percent of less than about 10% and greater than about 0.5% and amean hole diameter of less than about 775 nm and greater than about 5nm. In some embodiments region 34 has a regional void area percent ofless than about 6% and greater than about 0.5% and a mean hole diameterof less than about 300 nm and greater than about 5 nm.

FIG. 3 is a plot of the effective refractive index Δ vs. radius r,similar to the effective refractive index plot included in FIG. 2, foran example embodiment of a refractive index profile for multi-modeversion of nano-engineered fiber 10. Here, the reference refractiveindex n_(c) for the effective index calculation is the average for innerannular cladding 32. The wavelength is 850 nm.

Core region 20 has a continuously varying positive effective refractiveindex Δ₁ with a maximum Δ_(1MAX) at r=0 (i.e., at centerline 16). Outerannular cladding 36 has a substantially constant effective refractiveindex Δ₄, and in an example embodiment Δ₄=Δ₂=0%. Hole-containing region34 has a depressed index Δ₃.

In some embodiments, the inner annular cladding 32 has a relativerefractive index Δ₂ having a maximum value Δ_(2MAX)<0.05%, and−0.5%<A_(2MAX)<0.05%. In an example embodiment, the effective refractiveindex Δ₃ of hole-containing region 34 is the same as Δ₂ at radius R₂(i.e., Δ₂(R₂)=Δ₃(R₂)).

In some embodiments, the outer annular portion 36 has a relativerefractive index Δ₄ having a maximum value Δ_(4MAX)<0.05%, while inother example embodiments, −0.05%<Δ_(4MAX)<0.05%. In an exampleembodiment, Δ₄(R₃)=Δ₃(R₃).

In some embodiments, the inner annular cladding region 32 comprises puresilica. In some embodiments, outer annular cladding region 36 comprisespure silica. In some embodiments, the depressed-index hole-containingregion 34 comprises pure silica with a plurality of airlines 40.Preferably, the minimum relative refractive index, or average effectiverelative refractive index Δ₃, such as taking into account the presenceof any airlines, of the depressed-index annular portion 34 preferablysatisfies Δ₃<−0.1%. In example embodiments, airlines 40 contain one ormore gases, such as argon, nitrogen, or oxygen, or the airlines containa vacuum with substantially no gas; regardless of the presence orabsence of any gas, the effective refractive index Δ₃ in the annularportion 34 is lowered due to the presence of airlines 40.

As discussed above, airlines 40 can be randomly or non-periodicallydisposed in the annular portion 34 of cladding 30, and in otherembodiments, the airlines are disposed periodically. In someembodiments, the plurality of airlines 40 comprises a plurality ofnon-periodically disposed airlines and a plurality of periodicallydisposed airlines. Alternatively, or in addition, the depressed index ofannular hole-containing region 34 can also be provided by downdopingthis region (such as with fluorine) or updoping one or more of thecladding regions 32 and 36 and/or the core 20, wherein thedepressed-index hole-containing region 34 is, for example, pure silicaor silica that is not doped as heavily as the inner annular claddingregion 32.

Preferably, radius R₁>4 μm. In some embodiments, the minimum relativerefractive index Δ_(3MIN)<−0.10%; in other embodiments, Δ_(3MIN)<−0.20%;in still other embodiments, Δ_(3MIN)<−0.30%; in yet other embodiments,Δ_(3MIN)<−0.40%.

In an example embodiment, Δ1 _(MAX)≦2.0%, more preferably Δ1_(MAX)≦1.0%, even more preferably Δ1 _(MAX)<1.0%, and still morepreferably Δ1 _(MAX)≦0.8%; in some embodiments 0.4%≦Δ1 _(MAX)≦1.0%, andin other embodiments 0.5%≦Δ1 _(MAX)≦0.75%.

In an example embodiment, the numerical aperture (NA) of optical fiber10 is given by NA₁₀=n sin θ₁₀ and is preferably greater than thenumerical aperture NA_(LS)=n sin θ_(LS) of an optical light source LSoptically coupled to an end 12 of nano-engineered fiber 10, as shown inFIG. 4. For example, the NA₁₀ of the optical fiber is preferably greaterthan the NA of a vertical-cavity surface-emitting laser (VCSEL) source.

Multimode nano-engineered fibers 10 are discussed in U.S. patentapplication Ser. No. 12/004,174, entitled “Bend-resistant multimodeoptical fiber,” filed on Dec. 20, 2007, and incorporated by referenceherein. The bandwidth of the multi-mode version of nano-engineered fiber10 varies inversely with the square of Δ_(1MAX). For example, amulti-mode optical fiber 10 with Δ_(1MAX)=0.5% can yield a bandwidth 16times greater than an otherwise identical multi-mode optical fiber 10with Δ1 _(MAX)=2.0%. For example, using the designs disclosed herein,fibers can been made which provide (a) a bandwidth of greater than 750MHz-km, more preferably greater than 1.0 GHz-km, and even morepreferably greater than 2.0 GHz-km, and most preferably greater than 3.0GHz-km at a wavelength of 850 nm. These high bandwidths can be achievedwhile still maintaining a 1 turn 10 mm diameter mandrel wrap attenuationincrease at a wavelength of 1550 nm, of less than 0.5 dB, morepreferably less than 0.3 dB, and most preferably less than 0.2 dB.Similarly, these high bandwidths which exhibit such impressive bendperformance at 1550 nm can also maintaining a 1 turn 10 mm diametermandrel wrap attenuation increase at a wavelength of 850 nm of less than1.5 dB, more preferably less than 1.0 dB, and most preferably less than0.62 dB. Such fibers can also exhibit a 1 turn 10 mm diameter mandrelwrap attenuation increase at a wavelength of 1550 nm, in dB, of lessthan or equal to the product of two times (1/Δ_(1MAX))².

In some embodiments, 12.5 μm≦R₁≦40 μm, i.e. diameter 2R₁, of core 20diameter is between about 25 and 80 μm. In other embodiments, R₁>20microns. In still other embodiments, R₁>22 microns. In yet otherembodiments, R₁>24 microns.

Nano-Engineered Fiber Connectorization

The present invention provides methods for collapsing airlines 40 in thecladding region 30 of nano-engineered fibers 10 so as to perform opticalfiber connectorization in a manner that minimizes the impact on the MFDfor single-mode fibers (or core diameter in the case of multi-modefibers) and/or the outer cladding diameter and that facilitates theconnectorization process by avoiding processing the fiber end directly.

Connectorization Methods

An example embodiment of a method for processing a nano-engineered fiber10 for connectorization is now described. With reference now to FIG. 5Aand FIG. 5B, the method includes preparing the optical fiber bystripping the buffer and/or coating layer 50 from the optical fiber overa region 100 at a mid-span location 102 to expose a length or section ofbare fiber 110 as shown in FIG. 5A. The fiber is then cut to form afiber end face 112 (FIG. 5B). Bare fiber 110 is then optionally cleanede.g., with isopropyl alcohol solvent. Bare fiber 110 needs to besufficiently long to allow for fiber installation into an opticalconnector, allowing the bare fiber to extend completely through thelength of a connector ferrule, as discussed further below. In an exampleembodiment, region 100 is preferably about 10 to 40 mm long. Airlines 40are shown schematically by the parallel dashed lines in bare fiber 110.

After stripping and cleaning, then with reference to FIG. 5C, localizedregion 100 of bare optical fiber 110 is subjected to localized heatsufficient to close airlines 40 in airline-containing region 34 (FIG. 2)to form an airline-free portion 130 an axial distance d (i.e., thedistance along the fiber) from fiber end face 112, wherein in an exampleembodiment d is at least 10 fiber diameters (i.e., d≧10D₁₀) and inanother example embodiment is at least 20 fiber diameters (i.e.,d≧20D₁₀). In an example embodiment, 10D₁₀≦d≦80D₁₀, while in anotherexample embodiment 20D₁₀≦d≦80D₁₀. In an example embodiment, d is atleast 5 mm (i.e., d≧5 mm) and in another example embodiment d is atleast 10 mm (i.e., d≧10 mm).

Localized heating may be generated in various ways including, but notlimited to, an electric arc generated between two electrodes (as is donein most fusion splicers), a heated filament, a flame or a laser, amongothers. In the present example embodiment, this is accomplished by usinga fusion splicer 120 that produces an electric arc 122 with a prescribedcurrent and a predetermined duration. A suitable fusion splicer for usein carrying out the method of the present invention is, for example, TheComing miniMASS® Fusion Splicer, available from Corning Cable Systems,Inc., Hickory, N.C. The portion of bare fiber 110 is positioned infusion splicer 120 such that the arc can be applied to the localizedregion 100 of the fiber, which will eventually be positioned at the endof a connector ferrule after the installation and polishing processesare completed.

The localized region 100 is subjected to an electric arc 122, whichheats the fiber and causes airlines 40 therein to collapse into thecladding material. The current for electric arc 122 is for example inthe range of about 12 mA to about 16 mA for a single fiber. Largercurrents would be used for multiple fibers, such as fiber ribbons. Aproper current setting should be great enough to collapse the airlineswithout damaging the fiber, such as melting and deforming.

In an example embodiment, at least some axial length of region 100 isheated to between approximately 2300 and 2600° K. More preferably, atleast some axial length region 100 is heated to between approximately2300 and 2600° K. for greater than 500 msec. Even more preferably, atleast some axial length of region 100 is heated to between approximately2300 and 2600° K. for greater than 500 msec and less than 10,000 msec.Less time will result in a smaller airline-free portion 130.

Once airlines 40 have been collapsed to form airline-free portion 130 inbare fiber 110, then with reference to FIG. 5D, the bare fiber isinserted into an input end 202 of a central bore 203 (see FIG. 5E) of aconnector ferrule 204 contained within an optical connector housing 200.The insertion is performed so that there is some length of bare fiber110 that protrudes beyond ferrule output end face 206. FIG. 5E is aclose-up view of ferrule end face 206 of FIG. 5D. Note that airline-freeportion 130 of bare fiber 110 is positioned at ferrule end face 206.Note that in an example embodiment, the fiber can first be inserted intoa ferrule and then the airlines collapses. One skilled in the art willrecognize that the order of the acts making up the method can be changedin a manner consistent with obtaining the final connectorized opticalfiber assembly 250, and that performing the method in the orderpresented constitutes just one example embodiment of the method.

As part of the connectorization process, bare fiber 110 is thenprecision cleaved as close as possible to ferrule output end face 206and within airline-free portion 130 so that the new fiber end face 112formed by the precision cleaving has no airlines 40 (i.e., is solid).Following this precision cleave step, the ferrule output end face 206and the new fiber end face 112 is buffed and polished using standardpolishing techniques known in the art so that the new fiber end face 112is co-planar with ferrule end face 206. Inserting the stub fiber 110having collapsed airlines into a connector prior to cleaving isadvantageous in that fiber end face 112 is protected from externalforces and contamination until after it has been mounted within theferrule.

The remaining connector parts (e.g., boot 210) are then added to orotherwise incorporated with connector housing 200 to form theconnectorized optical fiber assembly 250 as illustrated in thecross-sectional view of FIG. 5F. FIG. 6 is a close-up cross-sectionalview of ferrule end face 206 similar to FIG. 5E, illustrating howairlines 40 terminate to form airline-free portion 130 at fiber end face112. Length L is the length of airline-free portion 130 as measured fromthe new (i.e., cleaved) end face 112 formed when bare fiber 110 isarranged in ferrule 204. In an example embodiment, about 10 μm≦L≦about10,000 μm.

FIG. 5G and FIG. 5H are similar to FIG. 5B and FIG. 5C and illustrate anexample embodiment of the present invention where the airlines 40 arecollapsed in each of a plurality of bare fibers 110 in a fiber ribbon10R. In an example embodiment, this is accomplished using a singleelectric arc 122 that provides a proportionally greater amount of heatthan for a single fiber. The connectorization process to formconnectorized optical fiber assembly 250 for the fiber ribbon is asdescribed above, with connector ferrule 204 having a plurality of bores203.

FIG. 7A and FIG. 7B illustrate an example embodiment of the methodwherein airlines 40 are collapsed at mid-span location 100 prior tocleaving bare fiber 110. This approach is advantageous in that two barestub fibers 110 are created, each having an end face 112 with respectiveairline-free portions 130 at a distance d from their respective endfaces. Each of these stub fibers can be connectorized as describedabove.

FIG. 7C and FIG. 7D are similar to FIG. 7A and FIG. 7B and illustrate anexample embodiment of the present invention where the airlines 40 arecollapsed at a mid-span location 100 in each of a plurality of barefibers 110 in a fiber ribbon 10R. In an example embodiment, this isaccomplished using a single electric arc 122 that provides aproportionally greater amount of heat than for a single fiber. Theconnectorization process to form connectorized optical fiber assembly250 for the fiber ribbon is as described above, with connector ferrule204 having a plurality of bores 203.

MFD Considerations for Single-Mode (SM) Fiber

An important consideration in connectorizing nano-engineered fibers ishow the MFD is affected by forming airline-free portion 130 using themethods described above. Maintaining the MFD of the nano-engineeredfiber is important because mismatches in MFD between fibers causeattenuation when the two fibers are connected, e.g., via fusion splicingor via a fiber optic connector.

Table 1 below presents measured data for the change in MFD for anexample non-nano-engineered fiber in the form of Corning SMF-28e fibersubject to the electric arc method of the present invention. The data inTable 1 provide a baseline for the amount of change in MFD (i.e., the“ΔMFD”) that can happen even to a standard (i.e., non-nano-engineered)fiber when subject to the methods of the present invention.

TABLE 1 MFD Change for Corning SMF-28e Fiber MFD (μm) 1310 nm 1550 nm1625 nm Before Arc 9.18 10.36 10.77 After Arc 9.27 10.50 10.92 % Change(ΔMFD) 1.00 1.42 1.42

Table 1 indicates that very small changes on the order of approximately1% to 1.4% occur in MFD for SMF-28e fiber when an arc is applied to thefiber. This change can be attributed to small changes in the index ofrefraction profile of the fiber due to thermal diffusion of one or moredopants in the core region.

Table 2 below is similar to Table 1, and presents measured data for thechange in MFD for a nano-engineered fiber having a C/C ratio of 0.42. Inthis example embodiment D₁₀=125 microns, Δ₁=approximately 0.34%,R₁=approximately 4.5 microns, R₂=approximately 10.7 microns, region 34is comprised of approximately 200 holes having a mean diameter ofapproximately 230 nm and a maximum diameter of <700 nm,W₂₃=approximately 4 microns and Δ₂=Δ₄=approximately 0%, and the air-fillfor this fiber=approximately 0.1%. This fiber had a cable cutoff of 1260nm showing that this fiber was single-moded above 1260 nm.

Table 3 below is similar to Table 2, but for a nano-engineered fiberwith a C/C ratio of 0.33. In this example embodiment D₁₀=125 microns,Δ₁=approximately 0.34%, R₁=approximately 4.5 microns, R₂=approximately13.6 microns, region 34 is comprised of approximately 200 holes having amean diameter of approximately 230 nm and a maximum diameter of <700 nm,W₂₃=approximately 5 microns and Δ₂=Δ₄=approximately 0%, and the air-fillfor this fiber=approximately 0.1%. This fiber had a cable cutoff of 1240nm showing that this fiber was single-moded above 1240 nm.

TABLE 2 MFD Change for a Nano-engineered Fiber for C/C = 0.42 MFD (μm)1310 nm 1550 nm 1625 nm With airlines 8.61 9.62 9.91 Collapsed airlines9.21 10.97 11.61 % Change (ΔMFD) 6.52 12.31 14.64

TABLE 3 MFD Change for a Nano-engineered Fiber for C/C = 0.33 MFD (μm)1310 nm 1550 nm 1625 nm With airlines 8.88 10.12 10.55 Collapsedairlines 9.26 10.86 11.48 % Change (ΔMFD) 4.10 6.81 8.10

The data in Table 2 and Table 3 indicate that the C/C ratio has asignificant impact on the change in the MFD, wherein the higher C/Cratio leads to larger changes in the MFD than the smaller C/C ratio. TheMFD also increases with increasing wavelengths. The data shows that thehaving a C/C of approximately ≦0.42 gives an acceptable change ofapproximately ≦10% in MFD at some if not all wavelengths.

FIG. 8A is a plot of the relative power (dB) vs. fiber radius r (μm) fora nano-engineered fiber having a C/C ratio of 0.42, and FIG. 8B is aplot of the relative power (dB) vs. fiber radius r (μm) for anano-engineered fiber having a C/C ratio of 0.33. The power measurementswere taken at the airline-free portion as well as airline-containingportions of the fiber. The differences in the power curves represent theΔMFD. The results shown in FIG. 8A and FIG. 8B confirm that a lower C/Cratio is needed in order to preserve the MFD between theairline-containing and airline-free portions of the fiber.

In an example embodiment, the ΔMFD is less than or equal to about 20%,more preferably less than or equal to about 15%, more preferably lessthan or equal to about 10%, and even more preferably less than or equalto 5%.

FIG. 9 plots simulated (modeled) data of the ΔMFD (in μm) versus the C/Cratio for a 125 μm nano-engineered fiber 10 that is single-mode aboveλ=1260 nm, and wherein ΔMFD is the difference between the MFD in theunprocessed nano-engineered portion of the fiber as compared to the MFDin the processed airline-free portion 130 formed using the abovemethods. The plot of FIG. 9 includes modeled data for the wavelengths1310 nm (curve with diamonds), 1550 nm (curve with squares) and 1625 nm(curve with triangles) and represents the theoretical limits of ΔMFD asa function of the C/C ratio and as a function of wavelength fornano-engineered fibers 10. The plot shows that the ΔMFD increases withan increasing the C/C ratio and increases with an increasing wavelength.

The inventors have confirmed through the modeling of FIG. 9 that a ΔMFDof less than 0.5 μm (i.e., <6% of an approximately a 8.4 μm MFD atλ=1310 nm) requires a C/C ratio of less than 0.37 for wavelengths up to1625 nm (see dashed line in plot). The plot of FIG. 9 also indicatesthat ΔMFDs as small as 0.27 μm (i.e., a ΔMFD of about 2.7% for a MFDabout 10 μm at λ=1625 μm), as small as 0.17 μm (i.e., a ΔMFD of about1.7% for a MFD about 10 μm at λ=1550 μm), and as small as about 0.03 μm(i.e., a ΔMFD of about 0.4% for a MFD about 8.4 μm at λ=1310 μm) areachievable using the present invention.

Fiber Diameter Considerations

As discussed above, nano-engineered fibers 10 to which the methods ofthe present invention apply have a relatively low air-fill percent ofless than 1% and is usually about 0.02% to about 0.2%, in contrast tophotonic crystal fibers, which have an air-fill percent of 5% to 20%,i.e., at least five times greater, and usually about two orders ofmagnitude greater. Thus, it is the small air-fill percent of thenano-engineered fibers considered herein that allows the fibers toretain their circularity and nominally their original size when theairlines are collapsed. This allows the processed fibers to remaincompliant with the ITU-T G.652 standard wherein the (bare) fiber has adiameter D₁₀=125 μm+/−1 micron for proper connectorization.

In contrast, a photonic crystal fiber, after collapsing the air holestherein, has a diameter change far greater than ±1 micron, and thus isnot ITU-T G.652 compliant for connectorization. Assuming that therelative air-fill percents correspond to the amount of fiber diameterchange, then a photonic crystal fiber undergoes a diameter change of atleast about 5× that of a nano-engineered fiber, and more typically about100×. Thus, an overall change in D₁₀ for a 125 μm fiber of less than 0.6μm, which would be acceptable for connectorizing a nano-engineered fiber10, would translate into a change of at least 2.5 μm and more typicallya change of about 5 to 50 μm when the method is applied to a photoniccrystal fiber—a change that would be deemed unacceptable for theconnectorization contemplated by the present invention.

The methods of the present invention are able to collapse airlines 40 innano-engineered fiber 10 while retaining substantially the samecross-sectional diameter D₁₀ and circularity, making the fibers andmethods advantageous for mounting the processed nano-engineered fiberwithin a ferrule in the course of forming a connectorized fiber.

In an example embodiment, the change ΔD₁₀ in the diameter D₁₀ of fiber10 at airline-free region 130 as compared to the other non-processed(i.e., airline-inclusive) regions of the fiber is less than or equal to1% (0.125 μm), more preferable less than or equal to 0.50% (0.625 μm),even more preferably less than or equal to 0.24% (0.30 μm), and evenmore preferably less than or equal to 0.08% (0.10 μm). The numbers inparenthesis are the actual percentage values for a 125 μm diameterfiber.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Thus, itis intended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A method of connectorizing an optical fiber comprising: providing anano-engineered fiber having a core, a cladding with non-periodicallydisposed airlines, and at least one of a coating and a buffer; strippinga length of the at least one of the coating and the buffer to expose amid-span portion of the optical fiber; selectively applying apredetermined amount of energy to the mid-span portion sufficient tocollapse the airlines at the mid-span portion to form an airline-freepotion thereat; mounting the optical fiber within a connector ferrulehaving a ferrule end face so that the airline-free potion is arranged atthe ferrule end face and so that a portion of the fiber protrudes beyondthe ferrule end face; and cleaving the optical fiber through theairline-free portion at or near the ferrule end face to provide a fiberend face having no airlines.
 2. The method according to claim 1,including polishing the fiber end face to lie in an approximately commonplane with the ferrule end face.
 3. The method according to claim 1,wherein the optical fiber has an air-fill percent less than about 1% andgreater than about 0.02%.
 4. The method according to claim 1, whereinthe optical fiber has an air-fill percent less than about 0.7% andgreater than about 0.02%.
 5. The method according to claim 1, whereinthe optical fiber has an air-fill percent less than about 0.2% andgreater than about 0.02%.
 6. The method according to claim 1, whereinthe optical fiber has an average airline size less than about 0.3microns and greater than 0.005 microns in cross-sectional diameter. 7.The method according to claim 1, wherein the stripped optical fiber hasa cross-sectional diameter that does not change by more than about 1%after the airlines are collapsed.
 8. The method according to claim 1,including providing the predetermined amount of energy in the form of anelectric arc having an associated current between about 12 mA and about16 mA.
 9. The method of claim 1, including: providing a plurality ofsaid nano-engineered fibers in the form of a ribbon; and selectivelyapplying said predetermined amount of energy to the mid-span portions ofthe fibers making up the ribbon, said amount of energy being sufficientto collapse the airlines at the mid-span portions to form respectiveairline-free portions thereat.
 10. The method according to claim 1,wherein the energy is supplied from at least one of a fusion splicer, aflame, a filament, and a laser.
 11. The method of claim 1, wherein thestripped fiber has a fiber diameter, and further including forming theairline-free portion at least ten fiber diameters from the fiber endface that exists prior to said cleaving operation.
 12. The method ofclaim 11, including forming the airline- free portion in the range fromten fiber diameters to eighty fiber diameters from said fiber end face.13. The method of claim 2, wherein the airline-free portion has a lengthL as measured from the cleaved fiber end face of about 10 μm≦L≦about10,000 μm.
 14. The method of claim 1, including performing the actstherein recited in the order in which they are presented beginning withproviding a nano-engineered fiber having a core through and includingthe cleaving step.
 15. A method of connectorizing an optical fiber,comprising: providing a nano-engireered fiber having a core, a claddingwith non-periodically disposed airlines, and at least one of a coatingand a buffer, wherein the optical fiber has a core/cladding ratio of0.42 or less and has associated therewith a first mode field diameter(MFD) at a given wavelength; stripping a length of the at least one ofthe coating and the buffer to expose a mid-span portion of the opticalfiber; selectively applying a predetermined amount of energy to themid-span portion sufficient to collapse the airlines at the mid-spanportion to form an airline-free portion thereat, wherein saidairline-free portion has associated therewith a second MFD at said givenwavelength, and wherein the change between said first MFD and saidsecond MFD is 20% or less; mounting the optical fiber within a connectorferrule so that a portion of the fiber protrudes beyond the ferrule endface, with the airline-free portion positioned at the ferrule end face;cleaving the optical fiber through the mid-span airline-free portion ator near the ferrule end face to provide a fiber end face having noairlines.
 16. The method according to claim 15, further includingpolishing the fiber end face and ferrule end face.
 17. The methodaccording to claim 15, wherein the optical fiber has an air-fill percentless than about 1%.
 18. The method according to claim 15, wherein theoptical fiber has an air-fill percent less than about 0.7%.
 19. Themethod according to claim 15, wherein the optical fiber has an air-fillpercent less than about 0.2%.
 20. The method of claim 15, wherein thestripped fiber has a fiber diameter, and further including forming theairline-free portion at least ten fiber diameters from the fiber endface that exists prior to said cleaving operation.
 21. The methodaccording to claim 15, wherein the energy is supplied from at least oneof a fusion splicer, a flame, a filament, and a laser.
 22. The method ofclaim 15, wherein the airline-free portion has a length L as measuredfrom the cleaved fiber end face of 10 μm≦L≦about 10,000 μm.
 23. Themethod of claim 15, including performing the acts therein in the orderpresented.
 24. A method of connectorizing an optical fiber, comprising:providing a nano-engineered fiber having a core, a cladding withnon-periodically disposed airlines and an outer diameter, and at leastone of a coating and a buffer; stripping a length of the at least one ofthe coating and the buffer to expose a mid-span portion of the opticalfiber; and selectively applying a predetermined amount of energy to themid-span portion sufficient to collapse the airlines at the mid-spanportion to form an airline-free portion thereat, wherein the claddingouter diameter changes by no more than 1% in the airline-free portion.25. The method of claim 24, further including mounting the optical fiberwithin a connector ferrule so that a portion of the fiber protrudesbeyond the ferrule end face with the airline-free portion positioned atthe ferrule end face.
 26. The method of claim 25, further includingcleaving the optical fiber through the mid-span airline-free portion ator near the ferrule end face to provide a fiber end face having noairlines.
 27. The method of claim 25, wherein the airline-free portionhas a length L as measured from the cleaved fiber end face of 10μm≦L≦about 10,000 μm.
 28. The method of claim 25, wherein there ispolishing the fiber end face.
 29. The method of claim 24, wherein thecladding outer diameter is 125 μm, and wherein the cladding outerdiameter changes by no more than +/−1μm in the airline-free portion. 30.The method of claim 24, further including forming the airline freeportion at least ten fiber diameters from the fiber end face that existsprior to said cleaving operation.
 31. The method according to claim 24,wherein the energy is supplied from at least one of a fusion splicer, aflame, a filament, and a laser.
 32. The method according to claim 24,wherein the optical fiber has an average airline size less than about0.3 microns and greater than 0.005 microns in cross-sectional diameter.33. The method of claim 24, including performing the acts therein in theorder presented.
 34. A connectorized nano-engineered optical fiberassembly, comprising: a connector ferrule having at least one bore andan end face; at least one nano-engineered fiber having a bare fibersection arranged in the at least one bore and having a core, a claddingwith non-periodically disposed airlines formed therein, a bare-fiberdiameter, and an end face formed by polishing to be coplanar with theferrule end face; and wherein the bare fiber section includes anairline-free portion that includes the bare fiber section end face thathas an airline-free bare-fiber diameter substantially the same as theairline-inclusive bare-fiber diameter.
 35. The assembly of claim 34,wherein the airline-free bare-fiber diameter is within about 1% of theairline-inclusive bare-fiber diameter.
 36. The assembly of claim 34,wherein the airline-inclusive bare fiber diameter is 125 μm, and whereinthe airline-free bare-fiber diameter changes by no more than +/−1 μmfrom the airline-inclusive bare-fiber diameter.
 37. The assembly ofclaim 34, wherein the nano-engineered fiber has a correspondingmode-field diameter (MFD) at a pre-determined wavelength, and whereinthe MFD does not change by more than 20% within the connector ferrule.38. The assembly of claim 34, wherein the nano-engineered fiber has acorresponding mode-field diameter (MFD) at a pre-determined wavelength,and wherein the MFD does not change by more than 5% within the connectorferrule.
 39. The assembly of claim 34, wherein the optical fiber has anair-fill percent less than about 1%.
 40. The assembly of claim 34,wherein the optical fiber has an air-fill percent less than about 0.7%.41. The assembly of claim 34, wherein the optical fiber has an air-fillpercent less than about 0.2%.
 42. The assembly of claim 34, wherein theairline-free portion has a length L as measured from the fiber end faceof 10 μm≦L≦about 10,000 μm.
 43. The assembly of claim 34, including: aplurality of ferrule bores; and a plurality of optical fibers arrangedone in each ferrule bore.
 44. A connectorized nano-engineered opticalfiber product formed by the process of: providing a nano-engineeredfiber having a core, a cladding with non periodically disposed airlines,an airline-inclusive bare-fiber diameter and at least one of a coatingand a buffer; stripping a length of the at least one of the coating andthe buffer to expose a mid-span portion of the optical fiber;selectively applying a predetermined amount of energy to the mid-spanportion sufficient to collapse the airlines at the mid-span portion toform an airline-free portion thereat having a corresponding airline-freebare-fiber diameter that is within 1% of the airline-inclusivebare-fiber diameter; mounting the optical fiber within a connectorferrule having a ferrule end face so that the airline-free portion isarranged at the ferrule end face and so that a portion of the fiberprotrudes beyond the ferrule end face; cleaving the optical fiberthrough the airline-free portion at or near the ferrule end face toprovide a fiber end face having no airlines; and polishing the fiber endface to lie in the same plane as the ferrule end face.
 45. The opticalfiber product of claim 44, wherein the airline inclusive bare-fiberdiameter is 125 μm, and wherein the airline-free bare-fiber diameter iswithin +/−1 μm of the airline-inclusive bare-fiber diameter.
 46. Theoptical fiber product of claim 44, wherein the process further includes:forming the airline-free portion at a distance of between ten and eightyairline-inclusive bare-fiber diameters from the fiber end face thatexists prior to said cleaving operation.
 47. The optical fiber productof claim 44, wherein the airline-free portion has a length L as measuredfrom the cleaved fiber end face of 10 μm≦L≦about 10,000 μm.
 48. Theoptical fiber product of claim 44, wherein the process further includes:providing the energy from at least one of a fusion splicer, a flame, afilament, and a laser.
 49. The optical fiber product of claim 44,wherein the nano-engineered fiber has a corresponding mode-fielddiameter (MFD) at a pre-determined wavelength, and wherein the MFD doesnot change by more than 20% within the connector ferrule.
 50. Theoptical fiber product of claim 44, wherein the nano-engineered fiber hasa corresponding mode-field diameter (MFD) at a pre-determinedwavelength, and wherein the MFD does not change by more than 5% withinthe connector ferrule.
 51. The optical fiber product of claim 44,wherein the nano-engineered fiber has an air-fill percent less thanabout 1% and greater than 0.02%.
 52. The optical fiber product of claim44, wherein the nano-engineered fiber has an air-fill percent less thanabout 0.7% and greater than 0.02%.
 53. The optical fiber product ofclaim 44, wherein the nano-engineered fiber has an air-fill percent lessthan about 0.2% and greater than 0.02%.
 54. The optical fiber product ofclaim 44, including performing the acts of the process in the orderpresented.